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Early involvement for large commercial building construction

12 July 2012

Due for completion this year, the Co-operative Group head office will be one of Manchester's largest commercial buildings, here Peter Buckley MIFireE and Stuart Martin explain how early fire service involvement helped enhanced the design:

Integrating approving authorities - fire services and building control - into the early development stages of a building's design is becoming more common practice, and is increasingly being viewed as one of the most effective ways of reducing risk in large and complex buildings prior to occupancy.

This was illustrated in one recent project in Manchester, where the early involvement of the fire authority and the city council building control helped enhance the fire engineering design for the Co-operative Group head office - the city's largest new development, due for completion this autumn.

Flagship Project

Valued at £100 million, the Co-operative building - located in the city centre directly opposite the CIS Tower - is a flagship development for the multi-faceted organisation. At 325,000sq ft [30,200m2], it is one of the biggest commercial office buildings in Manchester.

With a footprint of 4,000m2, the building comprises 14 floors of office accommodation and a central 65m atrium, which provides an architectural focal point for the interior. Ten of the 14 floors are open to the atrium, with a semi-basement and full basement beneath.

The major challenge for the design team - including fire consultant Buro Happold FEDRA - was developing a technical solution for 200,000m3 of building volume open to the atrium, to ensure that, in the event of an incident, fire and smoke spread through this space does not compromise the fire and life safety requirements.

A principal of natural ventilation has been adopted for the environmental design. Air is displaced through the raised access floor and flows through the office to the atrium, where it is assisted out of the building at the top. For those floors which are separated from the atrium, the air in the environment passes through vents in the office/atrium separating walls.

Early Consultation

Historically, consultations and negotiations between design teams and approving authorities take place when the design is progressing through the detailed design stage. This approach is adopted where the majority of the design decisions have been made and the building fits within the framework of conventional design codes.

For the new Co-operative building, such an approach was not considered appropriate because the design team and approving authorities needed to agree key parameters for the fire engineered elements of the design, which would ultimately influence the building's overall design. Greater Manchester Fire and Rescue Service (GMFRS) and Manchester City Council Building Control (MCCBC) were first introduced to the scheme in early 2010, when the design was still at the outline stage.

For such a large and complex building, it is not unusual for building control to appoint separate sub consultants to comment on fire engineered designs. However, in this case, the GMFRS fire engineering team was asked to take the lead on behalf of MCCBC in liaising with the designers, checking the design proposals and commenting on their suitability.

Defining the ChallengeThe design for the building follows the principles of BS 9999: Code of practice for fire safety in the design, management and use of buildings. The fire consultants could have simply followed the recommendations for atria set out in BS 9999, but these did not deliver the architectural and client aspirations for the connectivity of the atrium and office areas.

Furthermore, because the building is naturally ventilated through the atrium, the BS 9999 recommendations for a storey smoke extract system conflicted with the environmental design aspirations for the scheme. Therefore, the design solution was to provide mechanical smoke extract at the top of the atrium. Fire engineering - in the form of computational fire dynamics (CFD) fire and smoke modelling - was essential in determining the influences that the atrium would have on the life safety design of the building.

Fire and Smoke Analysis

The curving geometry of the external envelope and the interior spaces associated with the atrium, in conjunction with the volume of the building, presented a challenge in determining which modelling tools to adopt. Fire Dynamics Simulator (FDS), developed by the US National Institute of Standards and Technology, is the most common tool used by designers for analysing fire and smoke spread in modern buildings. However, the view was taken that, due to the scale of the building and its curving geometry, a more sophisticated tool was required.

Since the design team was evaluating environmental conditions in the building using ANSYS CFX 12.0 (CFX), and this suite of software can also handle fire and smoke modelling, it was agreed that this tool would be used. The key benefits of using CFX for CFD analysis on this particular project include its ability to model complex geometry, including non-rectilinear forms, and to handle large-scale models within reasonable computational times. CFX has a successful track record for fire and smoke modelling, having been used by the UK Health and Safety Laboratory for modelling the 1987 King's Cross underground fire and the 1996 blaze in the Channel Tunnel. It was also peer tested by the Department for Transport, Local Government and the Regions.

Modelling Essentials

At the time of design, GMFRS lead fire engineer Dr Jim Marsden, who has a detailed knowledge of CFD, undertook the role of adjudicator for the fire engineering element of the scheme. Over a period of six months, he worked closely with Dr Gary Walker of Buro Happold FEDRA on the CFD.

The two parties agreed:
• The fundamentals of modelling with CFX (suitability of the model, geometry, mesh sizes)
• The various input parameters that would be used (fire area, fire load, soot yields, sprinkler criteria, temperatures for summer and winter)
• The scenarios that would be considered (fire locations, summer versus winter, sprinklered and unsprinklered fires)
• The acceptance criteria against which the outputs could be assessed.

Air Supply

Low-level inlet air is essential for a functional smoke control system. The challenge for the designers on the Co-operative project was to provide sufficient paths for inlet air to a building which unusually has very limited opportunity for openings at low level. The technical solution adopted is to use a combination of doors/louvres at the main entrance, and mechanically assisted inlet air which provides the building's primary environmental air supply. This mechanical system delivers over 60% of the required inlet air volume.

Air is drawn from the public realm in front of the building and ducted 30m in two large earth tubes, from where it is displaced into the atrium at first-floor level. The CFD modelling needed to include an assessment of the forced air supply, to ensure that it did not adversely disrupt the smoke plumes by redirecting smoke towards upper storey escape routes or on to the open floors.

Ruling Out Sprinklers

Modelling potential fires within an atrium often raises a discussion point in fire engineering schemes. During the early design stages of some projects, clients have not considered how they will use the space and it is easy for consultants to place a limit on fire loading that building users find hard to interpret or manage.

Since the atrium at the Co-operative building is 65m at its highest point, the provision of a sprinkler system was not deemed a practical option. The sprinkler heads would be unlikely to operate in the event of an incident due to the distance between the sprinkler head and the fire seat. To address this, sensitivity studies carried out by the designers considered unsprinklered fires located within the at ground floor level, which included a displaying a car and a 17m Christmas tree.

After extensive modelling, the design evaluations demonstrated that 120m3/s of mechanical extract, in conjunction with the hybrid natural/mechanical make-up air at low level, would deliver a range of tenable solutions in the building for both escape and firefighting.

Pre-planning Benefits

The fire engineering team at GMFRS positively encourages the involvement of approving authorities in design discussions at the earliest opportunity. This approach benefits all concerned, since any problems can be resolved early in the design phase. In the Co-operative project, the initial involvement of the fire service and building control will help ensure greater business continuity and improved community safety throughout the life of the building.

Moreover, in line with GMFRS's operational strategy, information on the building will be collected to assist in incident pre-planning and the development of major incident plans, with relevant data recorded via its Operational Intelligence System and made available to fire incident commanders en route to an incident via the mobile data terminals on fire appliances.